any of a diverse group of microorganisms capable of metabolizing sulfur and its compounds and important in the sulfur cycle (q.v.) in nature. Some of the common sulfur substances that are used by these bacteria as an energy source are hydrogen sulfide (H₂S), sulfur, and thiosulfate (S₂O₃^2-). The final product of sulfur oxidation is sulfate (SO₄^2-).

Thiobacillus, widespread in marine and terrestrial habitats, oxidizes sulfur, producing sulfates useful to plants; in deep ground deposits it generates sulfuric acid, which dissolves metals in mines but also corrodes concrete and steel. Desulfovibrio desulficans reduces sulfates in waterlogged soils and sewage to hydrogen sulfide, a gas with the rotten egg odour so common to such places. Thiothrix, common in sulfur springs and in sewage, and Sulfolobus, confined to sulfur-rich hot springs, transform hydrogen sulfide to elemental sulfur.

Many species in the families Chromatiaceae (purple sulfur bacteria) and Chlorobiaceae (green sulfur bacteria) utilize energy from light in an oxygen-free environment to transform sulfur and its compounds to sulfates.